1. Field of the Invention
This invention relates to the manufacture of one-piece composite parts including wings and wing shells.
2. Description of the Related Art
Known processes for fabricating castable composite parts are very complicated and expensive. A large portion of the complexity and expense is associated with manufacturing related molds. Nearly any part can be constructed as a composite part by various production methods such as filament winding, tape placement, overbraid, chop fiber roving, coating, painting, dripping, hand lay up, resin soaked, or other composite processing technique and curing process.
When these parts are manufactured using a form or mandrel, there is typically a problem with removing the form (mandrel) from the finished part. For very simple parts, the form can be “designed to draw” e.g. the form can be removed from the one-piece composite part by simply drawing (pulling) the form out of the part. For more complex parts such as a composite valve 6 as shown in FIG. 1, the form 8 does not draw. One solution is to sacrifice or destroy the form upon removal from the finished part. Another solution is to disassemble the form and remove the segments from the part. Another solution is to create a form that remains part of the final composite part. Yet another solution is to use an inflatable form that can be removed by deflating the form after the composite part is created.
U.S. Pat. No. 7,422,714 entitled “Method Of Using A Shape Memory Material As A Mandrel For Composite Part Manufacturing” is a version of an inflatable mandrel and includes the steps of “providing a pre-formed tube of Shape Memory Polymer (SMP), using a mold to deform the SMP tube to replicate the mold forming a SMP mandrel, filament winding resin soaked fibers around the deformed SMP mandrel, curing the resin, causing the deformed SMP Mandrel to return to its original smaller shape, and quickly and easily removing the SMP tube from the composite part.” (see col. 4, lines 43-50). More particularly uncured SMP is molded to form the tube and cured. The “tube” being the memorized shape of the SMP. The tube is placed into a mold of the composite part, heated above its glass transition temperature at which point the SMP transforms from a rigid substance to an elastic, flexible and soft substance, “inflated” into replicate the interior of the mold and allowed to cool to below its glass transition temperature at which point the SMP transforms back to a rigid substance. The rigid deformed SMP is removed from the mold and is ready for filament winding (col. 6, lines 37-50). Once the composite part is laid up and cured on the exterior of the deformed SMP, the SMP is heated to above its glass transition temperature inducing the deformed SMP mandrel to relax to its memorized shape (not the inflated mandrel shape). The tube is then removed from the composite part. (col. 7, lines 21-39) The “tube” can be reused to form the same or different composite parts. However, the tube must be reinflated to the desired mandrel shape each time. This process requires a SMP that can deform from the memorized blank shape (tube) to the desired mandrel shape.
Air vehicles ranging from unmanned air vehicles (UAVs) of a few pounds to cruise missiles up to 10,000 pounds require strong yet lightweight and inexpensive wings. These types of air vehicles may place high loads on the wings and require the capability to maneuver rapidly. To reduce the overall cost of the air vehicles a manufacturing process for low-cost one-piece composite wings is needed.
As shown in FIG. 2, a wing 10 can be described by its length 12 (“semi-span”) measured from the “root” 14 to the tip 16, width 18 (“chord length”) measured at the root from the leading edge 20 to the trailing edge 22 and cross-section 24 as well as its taper 26, twist 28 and camber 30. “Taper” indicates the rate of change of the chord length 18 along the half-span of the wing from the root where the wing attaches to the airframe to the tip of the wing. Wings are tapered to control the distribution of lift along the wing span. “Twist” indicates the rotation of the cross-section 24 along an axis 32 through the half-span of the wing. Twist is provided to avoid stalling the aircraft along the entire span of the wing, allowing the pilot or control system time to recover. “Camber” is the asymmetry between the top and the bottom curves 34, 36 of the wing in cross-section. Camber affects the lift and pitching moment of the wing. Camber may vary from wing root to wing tip.
A limited class of air vehicles such as model RC (radio controlled) hobby planes and other low cost air vehicles may use wings that have no taper, twist or camber. There are a few options for manufacturing these simple wings. One common approach is an extrusion process in which aluminum is forced through a die having the desired wing cross-section to extrude the wing. Foam may be extruded in a similar process and composite material layed up on the foam to form the wing. In an alternate process, the foam may be cut and the composite laid up. In this foam process, the foam remains inside the composite shell as part of the wing. In another approach, composite material is laid up on a reusable form (machined or molded from a rigid material such as aluminum or steel), which is then drawn from the wing shell. The form is “designed to draw” from the composite wing shell.
A more general class of air vehicles including UAVs, munitions, missiles, and other tube or pylon launched air vehicles demand greater performance and may use wings formed with taper, twist and/or camber. The industry standard for manufacturing such wings is to machine the wing from a solid block of aluminum. Similarly, a block of foam can be machined and composite material laid up to form a composite wing. In both processes, an expensive machining procedure is required to form each wing that wastes considerable materials. A wing with either twist or taper cannot be extruded. A wing with twist or taper and camber cannot be laid up on a reusable form because the form will not draw out and so cannot be removed. One could form pieces of the wing e.g. top and bottom on separate rigid forms and then assemble the pieces. However, this approach does not provide a one-piece wing shell, hence wing, requires additional assembly and potentially reduces the structural integrity of the wing.